Spleen and peripheral blood lymphocytes were found to play an important role in the pathogenesis of amyotrophic leukospongiosis (AL). At early stage of AL they are the sites of reproduction and accumulation of an unconventional virus; they also provide its dissemination in the body of infected animals. In lymph nodes and visceral organs the AL pathogen was detected in much lower quantities and only in the period of clinically manifest disease. The level of the complement was significantly decreased in the serum of animals with experimental AL. This decrease correlated with the development of clinico-morphological lesions and reached its maximum at the terminal stage of disease.